Multi-thickness current collector

ABSTRACT

A current collector in the form of a conductive substrate subjected to a special chemical etch to provide the current collector having a multi-thickness structure, is described. The multiple-thickness current collector structure provides an electrochemical cell with increased charge capacity per volume while enabling a robust weld connection thereto. The current collector has a frame and comprises within an inner perimeter of the frame, first strand structures that intersect second strand structures to provide a plurality of openings or interstices bordered by the strands. At least one tab portion having a thicker distal portion spaced from a thinner proximal tab portion that extends from an outer perimeter of the frame.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/467,966, filed on Aug. 25, 2014, now U.S. Pat. No. 9,455,448, whichclaims priority from U.S. Provisional Patent Application Ser. No.61/869,226, filed Aug. 23, 2013.

FIELD OF THE INVENTION

The present invention generally relates to the conversion of chemicalenergy to electrical energy. More particularly, the present inventionrelates to a conductive substrate that is used as a current collector inan electrochemical cell.

PRIOR ART

In general, it is desirable to maximize the amount of active cathode andanode material within a given volume of an electrochemical cell. Thus,increasing the amount of active electrode material provides for morematerial to react which, therefore, increases the capacity of anelectrochemical cell.

However, inactive components such as the anode and cathode currentcollectors, which are necessary for electrical conduction of anelectrode assembly, occupy a portion of the volume within the cell thatcould otherwise be occupied by additional active electrode materials.This is particularly the case for electrochemical cells having arelatively small volume. Given a particularly small sized cell,regaining even a fraction of a cubic inch of volume in active electrodematerial could be significantly beneficial in improving the useful lifeof a cell. Therefore, it is desired to construct an electrochemical cellsuch that the inactive components therewithin occupy a minimal amount ofvolume so that the amount of active electrode material can be maximized.Such a maximized amount of active electrode material provides for anelectrochemical cell with increased electrical energy capacity per unitcell volume.

Prior art current collectors are typically constructed having a uniformthickness. These prior art designs are not optimal because some of theuniform thickness of the current collector unnecessarily occupies volumewithin the cell which could otherwise be occupied by active electrodematerial. However, if the current collector is constructed having athickness that is too thin, there may not be a sufficient amount ofcollector material to provide a mechanically robust connection, such asvia a weld connection, to a terminal lead within the cell. The terminallead, which at least partially resides within the casing of a cell,provides an electrical connection between the current collector and anexternal electrical load powered by the cell. Thus, it is important thatthe mechanical connection between the current collector and the terminallead is mechanically robust such that it can withstand variousmechanical stresses without disconnecting over long periods of time.

The connection between the current collector and a terminal lead istypically formed by joining a tab portion that extends from a frame ofthe current collector to the lead by a weld connection, such as a laseror resistance weld connection. However, if the tab portion is too thin,it may be difficult to form a robust connection. For example, heat fromthe laser beam of the welding process may burn through the thickness ofthe tab portion. In addition, if the thickness of the tab portion is toothin, heat of the welding process may embrittle the metal and thus forma brittle joint that is not mechanically robust. Thus, it is importantthat the tab portion of the current collector be of a sufficientthickness to enable the formation of a robust mechanical connection to aterminal lead. Therefore, a current collector having a structurecomprising an optimized thickness profile is desired to allow for anincreased capacity as well as enabling a mechanically robust weldconnection.

The present invention therefore, provides a current collector having amore optimized design that achieves a balance between increasing thevolume of active electrode material without sacrificing the ability toprovide a robust weld connection. The current collector of the presentinvention utilizes a reduced amount of material at selected locations tothereby allow for a greater volume of active electrode material while atthe same time enabling the formation of a mechanically robust weldconnection thereto.

More specifically, the present invention provides a current collectorcomprising an active electrode material contact area with a relativelythin thickness and a thicker tab portion that extends therefrom. Therelatively thin active electrode material contact area of the currentcollector of the present invention allows for the incorporation ofadditional active electrode material within a given volume of anelectrochemical cell. In addition, the increased thickness of the tabportion of the current collector provides more material with which toform a robust mechanical connection, specifically, a mechanicalconnection between the current collector and a terminal lead positionedwithin the cell.

SUMMARY OF THE INVENTION

The present invention is, therefore, directed to a conductive substratethat serves as a current collector having a structure that provides animproved mechanically robust connection between a terminal lead of anelectrochemical cell and the current collector. At the same time, thepresent current collector allows for increased volumetric efficiency ofactive electrode material within a given electrochemical cell volume.This is achieved by constructing the current collector of the presentinvention with multiple thicknesses. Specifically, the current collectoris constructed having an active electrode grid portion with a relativelythin thickness and an outwardly extending connection tab portion, havinga greater thickness than that of the grid portion.

The grid portion of the current collector is designed to contact theactive electrode material and provide an electrical connection thereto.In addition, the grid portion is constructed having a relatively thinthickness that allows for an increased volume of active electrodematerial to be positioned within the cell and thus, as a result,increase the volumetric efficiency and capacity of the cell. As definedherein, “capacity” is the amount of electrical charge that is deliveredby a cell over a rated voltage. At least a portion of the tab portionthat extends outwardly from the grid portion of the current collector ispreferably constructed having a thickness that is more optimallydesigned for the formation of a weld connection. Thus, the structure ofthe current collector of the present invention is optimally designed toboth increase the volumetric efficiency and charge of an electrochemicalcell while at the same time providing a mechanically robust connectionof the current collector to a terminal lead or other electricalconnection within the cell case.

Specifically, the current collector is designed such that the distalportion of the tab has an increased thickness in comparison to theopposing proximal portion of the tab and the grid portion that contactsthe active electrode material. Increasing the thickness of the distalportion of the tab provides substantially more material with which toform a contact weld. Furthermore, reducing the thickness of the activeelectrode material contact area and proximal tab portion provides morevolume for additional active electrode material and activatingelectrolyte. In addition, the relatively thin proximal tab portionprovides the current collector with increased flexibility and freedom ofmotion. The current collector therefore, enhances volumetric efficiencyand capacity while at the same time provides for a flexible, robustmechanical connection within the cell.

When an electrochemical cell containing electrodes built with thepresent current collector is used to power an implantable medicaldevice, such as a pacemaker or cardiac monitoring device, there resultsreduced charging times and increased discharge capacity, therebyextending the medical device life. In addition, the current collector ofthe present invention allows for support flexibility in selection ofelectrode material type by optimizing the passive current collectormaterial volume.

The present current collector is preferably formed by chemically etchingselected surfaces of a blank sheet of metal. Multiple applications ofthe chemical etchant may be applied to selected surfaces of the metalblank such that the desired thickness of the grid structure and tabportion is achieved. Alternatively, the structure of the currentcollector may also be formed by machining or laser trimming a sheet ofmetal to the desired shape and size.

These and other objects of the present invention will becomeincreasingly more apparent to those skilled in the art by reference tothe following description and to the appended drawings.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front view of an embodiment of the current collector of thepresent invention.

FIG. 1A is an enlarged view of a tab that extends outwardly from thebody of the current collector shown in FIG. 1.

FIG. 1B is a cross-sectional view along line 1B-1B of FIG. 1A.

FIG. 2 is a side view of the current collector shown in FIG. 1.

FIG. 3 is a front view of an embodiment of a prior art currentcollector.

FIG. 3A is an enlarged view of a tab portion of the prior art currentcollector shown in FIG. 3.

FIG. 3B is a cross-sectional view along line 3B-3B of FIG. 3A.

FIG. 4 is a side view of the prior art current collector shown in FIG.3.

FIG. 5A is an enlarged view of an embodiment of a grid structure of thecurrent collector of the present invention.

FIG. 5B is an enlarged view of an alternate embodiment of a gridstructure of the current collector of the present invention.

FIG. 6 is an embodiment of a current collector structure of the presentinvention.

FIG. 7 is a side view of the current collector structure embodied inFIG. 6.

FIG. 8 illustrates a cross-sectional view of an embodiment of anelectrode utilizing the current collector of the present invention.

FIG. 9 is a perspective view of an embodiment of an electrode assemblyutilizing the current collector of the present invention.

FIG. 10 is an enlarged partially broken cross-sectional view of thedistal tab portion welded to a terminal lead.

FIG. 11 is a flow chart that illustrates an embodiment of a process ofcreating the current collector of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Turning now to the drawings, FIGS. 1, 1A, 1B, 2, 6 and 7 show anembodiment of a conductive substrate 10 according to the presentinvention. The conductive substrate 10 is particularly useful as acurrent collector in an electrochemical energy storage device, has agenerally elongated, rectangular shape provided with a perimeter frame12 and an integral interior reticulum or grid structure 14 bordered bythe frame 12. As illustrated, in FIGS. 5A and 5B, the frame 12 comprisesa frame width 16 that is defined by an exterior frame perimeter 18spaced from an interior frame perimeter 20. At least one tab 22 extendsfrom the frame 12, and in the preferred embodiment of the conductivesubstrate 10 is integral therewith. More specifically, the tab 22extends outwardly from the exterior frame perimeter 18. If desired, thetab 22 can be a separate part that is subsequently welded, fused orotherwise secured to the conductive substrate 10 at the frame 12thereof. The tab(s) 22 provide for connecting an electrode 24 (FIGS. 8and 9) comprising an active electrode material 26, such as an activeanode material and/or an active cathode material, and the conductivesubstrate 10 to a cell terminal 28 (FIG. 10) or other electricalconnection such as a case lid or case wall therewithin. Depending on theconstruction or configuration of the cell in which the substrate will beincorporated, the conductive substrate 10 may have a shape other thanthe rectangular one shown.

FIG. 1A illustrates an enlarged view of an embodiment of the tab 22 ofthe current collector 10 of the present invention. As shown, tab 22comprises a tab length 30 and a tab width 32 that extends aboutperpendicular from the tab length 30. The length 30 of the tab 22extends from a proximal tab portion 34 having a proximal tab end 36 to adistal tab portion 38 having a distal tab end 40. In a preferredembodiment, the tab length 30 may range from about 0.5 cm to about 5 cmwith the distal portion 38 of the tab 22 comprising from about 0.1 cm toabout 4 cm of the total tab length 30. Alternatively, the distal tabportion 38 may be constructed such that it comprises from about 5% toabout 95% of the total tab length 30. In a preferred embodiment, thewidth 32 of the tab 22 may range from about 0.01 mm to about 10 mm.

As illustrated, the proximal end 36 of the tab 22 extends outwardly fromthe exterior perimeter 18 of the frame 12 and the grid structure 14. Thedistal portion 38 of the tab 22 is designed to provide an electricalconnection within the cell, such as by physically joining andelectrically contacting a terminal lead 28 (FIG. 10) that is at leastpartially positioned within an electrochemical cell.

In a preferred embodiment, the distal portion 38 of the tab 22 isconstructed having a first thickness 42 (FIG. 1B) that is greater than asecond thickness 44 of the proximal portion 34 of the tab 22. The firstthickness 42 of the distal tab portion 38 extends perpendicular from thetab length 30 and is defined by opposing first and second major distaltab portion surfaces 43, 45 (FIG. 2). In that manner, a step 41 from thefirst thickness 42 to the second thickness 44 delineates the distal tabportion 38 from the proximal tab portion 34. The second thickness 44 ofthe proximal tab portion 34 also extends perpendicular from the tablength 30 and is defined by first and second major proximal tab portionsurfaces 47, 49 (FIG. 1B). In addition, the first thickness 42 of thedistal tab portion 38 is constructed such that it is greater than athird thickness 46 of the frame 12 (FIG. 2) and a fourth thickness 48 ofthe grid structure 14 (FIG. 8). The third thickness 46 of the frame isdefined by opposed first and second major frame surfaces 51, 53 (FIG. 2)and the fourth thickness 48 of the grid structure is defined by firstand second major grid surfaces 55, 57 (FIG. 8).

The thicker distal tab portion 38 provides for more material with whichto attach the current collector 10 to a terminal lead 28 (FIG. 10) of anelectrochemical cell. In addition, the relatively thinner proximal tabportion 34, frame 12 and grid structure 14 of the current collector 10allows for the incorporation of an increased volume of active electrodematerial 26 within the cell as the thinner proximal tab portion 34,frame 12 and grid structure 14 occupy less space within the cell. As aresult, an electrochemical cell (not shown) comprising the currentcollector 10 of the present invention has an increased volumetricefficiency and charge as compared to a similar cell constructed with thecurrent collector of the prior art having a uniform thickness. Forexample, an electrochemical cell constructed with the current collector10 of the present invention may exhibit an increase in volumetricefficiency and capacity, on a per unit cell volume basis, of betweenabout 2 percent to about 10 percent as compared to a similarelectrochemical cell constructed with a current collector of the priorart having a uniform thickness.

In addition, as illustrated in FIGS. 1, 1A, and 6, the proximal portion34 of the tab 22 may comprise a notch 35. As illustrated in FIG. 1A, thenotch 35 is preferably oriented about perpendicular to the tab length 30and extends within a portion of the second thickness 44 of the proximalportion 34 of the tab 22. More preferably, the notch 35 may reside atthe junction between the distal tab portion 38 and the proximal tabportion 34. The notch 35 enables increased flexibility and improvedhinging motion of the tab 22. As illustrated in FIG. 1A, the tab 22 maycomprise left and right notches 35A, 35B which respectively extendopposite each other into the second thickness 44 of the proximal tabportion 34.

In comparison, the current collector 50 of the prior art, illustrated inFIGS. 3, 3A, 3B and 4, is generally constructed having a uniformthickness. As shown in the side view of FIG. 4, the tab portion 52,frame 54 and grid portion 56 of the prior art current collector 50 isconstructed of a uniform thickness. As illustrated in FIGS. 3B and 4,the tab thickness 58 extending along the full length of tab 52 is thesame as the thickness 60 of the grid portion 56. Thus, when constructingelectrochemical cells utilizing the current collector 50 of the priorart, a trade-off between an increased volumetric efficiency and chargeand a robust weld connection is often made. For example, since thecurrent collector 50 of the prior art is constructed having a uniformthickness, it can either be constructed having a relatively thinthickness, which allows for an increased volume of active electrodematerial and thus an increase in volumetric efficiency and charge, butcompromises weld strength and durability. Alternatively, the currentcollector 50 of the prior art can be constructed having a greaterthickness which thus increases the strength and durability of the weldconnection, but sacrifices active electrode volume and capacity. Incomparison, the multi-thickness design of the current collector 10 ofthe present invention enables an increased electrode volume whileenabling a more robust current collector connection.

As illustrated in FIGS. 1, 5A and 5B, the grid structure 14 of thepresent current collector 10 preferably comprises a lattice constructionsurrounded by the interior perimeter 20 of the frame 12. In anembodiment, the grid structure 14 comprises first strand structures 62that intersect second strand structures 64 to provide a plurality ofopenings or interstices 66 bordered by the strands (FIGS. 5A and 5B).The strand structures 62, 64 intersect or join with each other atjunctions 68 thereby forming the grid structure 14 as an integral unit.The openings 66 of the grid structure 14 may be of a plurality ofnon-limiting shapes such as a diamond-shape as illustrated in FIGS. 1and 5A or a rectangular-shape as illustrated in FIG. 5B. In addition,the interstice 66 may comprise an opening of a shape selected from acircle, a triangle, an octagon, a hexagon or other polygon shape. It isfurther contemplated that the grid structure 14, itself may beconfigured of a shape selected from a circle, a triangle, an octagon, ahexagon or other polygon shape. The open area provided by theinterstices 66 may range from about 2 percent to about 90 percent of thetotal area surrounded by the interior perimeter 20 of the frame 12. Orthe conductive substrate 10 could be devoid of a grid structure. Inother words, there would not be any interstices. It is furthercontemplated that the lattice structure of the interior grid 14 may beconfigured in a plurality of non-limiting arrangements, examples ofwhich are illustrated in U.S. Pat. No. 8,741,487 to Duggan et al., whichis assigned to the assignee of the present invention and incorporatedherein by reference.

In a preferred embodiment, as illustrated in FIGS. 1B and 2, the firstthickness 42 of the distal portion 38 of the tab 22 may range from about0.01 mm to about 2.0 mm. The second thickness 44 of the proximal portion34 may range from 0.01 mm to about 1.5 mm. Alternatively, the secondthickness 44 of the proximal tab portion 34 may be constructed such thatit is about 25 to about 75 percent of the first thickness 42 of theproximal tab portion 34.

Furthermore, the third thickness 46 (FIG. 2) of the frame 12 may rangefrom about 0.01 mm to about 1.5 mm. Likewise, the fourth thickness 48(FIG. 8) of the grid structure 14 may be about 0.01 mm to about 1.5 mm.In an embodiment, the third thickness 46 of the frame 12 and the fourththickness 48 of the grid structure 14 may be constructed having aboutthe same thickness as the second thickness 44 of the proximal tabportion 34. In an alternate embodiment, the first thickness 42 of thedistal tab portion 38, the second thickness 44 of the proximal tabportion 34, the third thickness 46 of the frame 12 and the fourththickness 48 of the grid structure 14 may have different thicknesses.For example, the first thickness 42 of the distal tab portion 38 may beconstructed to be greater than the second thickness 44 of the proximaltab portion 34. The second thickness 44 of the proximal tab portion 34may be greater than the third thickness 46 of the frame 12 and the thirdthickness 46 of the frame 12 may be constructed to be greater than thefourth thickness 48 of the grid structure 14. In this particularembodiment, the thinner grid structure 14 of the current collector 10allows for an even greater volume of the active electrode material 26 tobe positioned within the cell.

FIGS. 6 and 7 illustrate an embodiment of a current collector structure70 in which a first current collector 72 and a second current collector74 of the present invention are joined together. As illustrated, thecurrent collector structure 70 comprises a tab junction 76 in which therespective tabs 22 of the first and second current collectors 72, 74 arejoined together. Specifically, the current collector structure 70 isconstructed such that the distal ends 40 of the respective distalportions 38 of the first and second current collectors 72, 74 are joinedtogether. As illustrated, the first and second current collectors 72, 74of the structure 70 are joined such that they are mirror images of eachother. The increased thickness of the respective distal tab portions 38of the current collectors 72, 74 provides the junction 76 with anincreased thickness that enables the construction of an electrodeassembly 81 with a robust weld connection as shown in FIG. 9. In thisembodiment, the tab junction 76 forms an anchor from which the opposingfirst and second current collectors 72, 74 hinge therefrom. This hingedesign allows for the two halves of the current collector structure 70to bend independently. In addition, the increased flexibility affordedby the distal tab junction 76 allows for construction of smaller cellsizes as the current collector is contorted and confined in a reducedvolume. The thicker distal tab portions 38 of the respective opposedfirst and second current collectors 72, 74 also increases the weldconnection durability and robustness.

FIG. 8 illustrates an embodiment of the current collector 10 of thepresent invention incorporated into an electrode 24. As shown, activeelectrode material 26 comprising either an active anode material and/oran active cathode material is contacted to the first major surface 55and/or the second major surface 57 of the grid structure 14 of thecurrent collector 10. As shown in this embodiment, a separator material78 may be wrapped around the active electrode material 26 sealing ittherewithin. As shown, the distal tab portion 38 extends outwardly fromthe electrode 24.

FIG. 9 illustrates an embodiment of an electrode assembly 81 comprisingthe current collector structure 70 of the present invention. As shown,the distal tab junction 76 is shown welded to an inner surface 80 of acasing lid 82. The distal tab junction 76 allows for a durable androbust weld connection 84 of the current collector structure 70 to theinner surface 80 of the casing lid 82. Alternatively, the distal tabportion 38 of a single current collector 10 may be welded to the insidesurface 80 of a case lid 82. The increased robustness of the weldconnection 84 between the current collector structure 70 and the casinglid 82 allows for increased movement and durability of the opposed firstand second current collectors 72, 74. Thus, an additional amount ofactive electrode material 26 may be positioned within a casing (notshown) of the cell.

FIG. 10 shows a cross-sectional view of an embodiment of a weldconnection 84 between the current collector 10 of the present inventionand a terminal lead 28 of an electrochemical cell. As shown, the distaltab portion 38 of the current collector 10 is shown welded to a terminallead 28.

According to the present invention, the current collector 10 of thepresent invention is fabricated by the controlled dissolution orcorrosion of a sheet-like or foil shaped workpiece through contact withan etchant in a chemical machining or photochemical machining process.In that respect, the conductive substrate 10 begins as conductive coilstock (not shown) having generally planar opposed major surfaces in anuncoiled, laid flat orientation. The coil stock preferably has athickness of about 0.001 to about 2 mm and is cut into sheets from whicha multiplicity of current collectors 10 or current collector structure70 will subsequently be fabricated in a batch operation. The cut sheetsare subjected to a precleaning process such as a chromic acid bath toremove scale and then run through a pumice slurry that serves to renderthe workpiece sheets having a clean condition, ready for processingafter being rinsed and dried.

A dry film resist or mask is then applied to selected portions ofsurface(s) of the workpiece to thereby protect the coated surfaces fromthe chemical action of the subsequent chemical machining orphotochemical machining process. As is well known by those skilled inthe art, the protective resist is inert to the etchant compounds, isable to withstand the heat from etching, adheres well to the workpieceand is easily and inexpensively removed after etching. The resist mustalso be tough enough to withstand handling; rigid enough to preventdrooping when undercut, yet scribe easily or spray cleanly. Numeroussynthetic or rubber-base resist materials are available in a widevariety of types and trade names.

To fabricate the conductive substrate 10 or current collector structure70, the resist is first applied to the first major surface 43 of thedistal tab portion 38 of the workpiece. A second coating of the resistmaterial may also be adhered to the second major surface 45 of thedistal tab portion 38 of the workpiece. Preferably, the resist isapplied to the workpiece as a photoresist by a photographic technique.Such a process begins with a photo-sensitive resist applied to either orboth major surfaces 43, 45 of the distal tab portion 38 followed by airdrying or oven baking the resist contacted workpiece.

Next, contact printing from a workpiece negative of the to be produceddistal tab portion 38 is followed by photographic development anddrying. The workpiece is next moved through an etchant bath or otherwisecontacted by the etchant solution such as by spraying. In the case of atitanium workpiece, for example, the etchant comprises a hydrogenfluoride/nitric acid solution. Those skilled in the art will readilyrecognize etchant solutions that are useful with other conductivesubstrate materials according to the present invention such asmolybdenum, tantalum, niobium, cobalt, nickel, stainless steel,tungsten, platinum, palladium, gold, silver, copper, chromium, vanadium,aluminum, zirconium, hafnium, zinc and iron, and the like, and mixturesand alloys thereof.

The workpiece with the applied resist pattern having the shape of thedistal tab portion 38 is contacted with the etchant for a period of timesufficient to etch away from each major surface a thickness of theworkpiece material such that the remaining thickness of the to be formedproximal tab portion 34, frame 12 and grid structure 14 of the currentcollector 10 or current collector structure 70 is less than that of thefirst thickness 42 of the distal tab portion 38. Those areas notprovided with resist on either major surface will be removed by theetchant chemical or solution of chemicals. In a preferred embodiment,the amount of material that is removed in this first etching stepestablishes the second thickness 44 of the proximal tab portion 34, thethird thickness 46 of the frame 12 and the fourth thickness 48 of thegrid structure 14.

After the thicknesses of the proximal tab portion 34, the frame 12 andthe grid structure 14 have been established by the first etchingprocess, a second application of resist is applied to the first majorsurfaces of the configuration of the proximal tab portion 34, the frame12 and the strand structures 62, 64. The resist may also be applied tothe second major surfaces of the proximal tab portion 34, the frame 12and the strand structures 62, 64 if desired.

As before, the resist is applied to the workpiece as a photoresist by aphotographic technique. Such a process begins with a photo-sensitiveresist applied to the entire area of each major surface of the workpiecefollowed by air drying or oven baking the resist contacted workpiece.Next, contact printing from a workpiece negative of the to-be-producedframe 12 and grid structure 14 is followed by photographic developmentand drying. The workpiece is next moved through an etchant bath orotherwise contacted by the etchant solution such as by spraying.

The workpiece with the applied resist pattern having the shape of theframe 12 surrounding the grid structure 14 is contacted with the etchantfor a period of time sufficient to etch away from each major surface athickness of the workpiece material. That way, those areas not providedwith resist on either major surface of the workpiece will be completelyremoved to provide the grid structure openings or interstices 66.

After the workpiece has been chemical machined to the desired extent toprovide the desired grid structure 14 (FIGS. 1, 5 and 5A) having theopenings 66 bordered by the frame 12 and the first and second strandstructures 62, 64, the resist material is removed in an aqueousstripping solution. After rinsing and inspection, the individualconductive substrates 10 or current collector structures 70 are cut orotherwise removed from the workpiece sheet and are ready forincorporation into an electrochemical energy storage device. Inparticular, the thusly formed conductive substrates can be used tofabricate either the anode or the cathode of a primary or secondaryelectrochemical cell or battery.

FIG. 11 is a flow chart that illustrates an embodiment of the steps of achemical machining process that may be utilized to fabricate the currentcollector 10. As shown in the flow chart, a substrate is first obtainedand cleaned. Secondly, a coating of resist material is applied to thedistal tab portion 38. The substrate is then chemically etched to formthe proximal tab portion 34 and the active electrode material contactareas comprising the frame 12 and grid structure 14, to a desiredthickness. Alternatively, if desired, a coating of resist material maybe applied to both the distal and proximal tab portions 38, 34. Thesubstrate is then subsequently chemically etched to form the activeelectrode material contact area to a desired thickness that is less thanthe distal and proximal tab portions 38, 34. Lastly, an additionalcoating of resist material is applied to the substrate to form the gridstructure 14 to a desired configuration. The substrate is then subjectedto a chemical etchant or solution of etchants to form the grid structureopenings 66. Additional examples of preferred chemical etchantprocessing methods are provided in U.S. Pat. No. 6,110,622 to Frysz etal., which is assigned to the assignee of the present invention andincorporated herein by reference.

As shown in FIGS. 1, 5A, 5B, and 6, in its finished form the conductivesubstrate 10 has substantially parallel first strand structures 62intersecting second strand structures 64. In a preferred embodiment, thefirst and second strand structures may be oriented from each other at anangle indicated at 65 of about 90 degrees to about 15 degrees (FIGS. 5A,5B). Each of the strands 62, 64 provide the first major surface of theconductive substrate 10 having a relatively smooth outer surfaceextending longitudinally along the strand 62, 64 length and joined tothe frame 12 where the resist was located. Alternatively, the firstmajor surface of the conductive substrate 10 may have a relatively roughouter surface.

As is readily apparent from the previous description, the strands 62, 64of substrate 10 are substantially co-planar with the respective firstand second major surfaces of the frame 12. If desired, the outersurfaces of the strands 62, 64 can be recessed somewhat from the firstand second major frame surfaces 51, 53, thereby providing the fourththickness 48 of the grid structure 14 being less than the thirdthickness 46 of the frame 12. Also, while the grid structures 14 inFIGS. 5A and 5B are shown having the respective strand structuresaligned parallel to each other, that is not necessary. Those skilled inthe art will understand that the strands 62, 64 need not be parallel butcan have a variety of shapes including wavy, sinusoidal, concentric, andzig-zag among a myriad of others. It is further contemplated that thefirst and second strand structures 62, 64 may be configured in a “basketweave” structure in which the first or second strands 62, 64 arerespectively positioned over each other. Examples of this “basket weave”grid structure 14 are provided in U.S. Pat. No. 6,110,622 to Frysz etal.

Examples of electrode active materials 26 that may be contacted to theconductive substrate 10 to provide an electrode (FIGS. 8 and 9)according to the present invention include metals, metal oxides, metalsulfides and mixed metal oxides. While not necessary, the electrodeactive material is preferably coupled with an alkali metal anode. Suchelectrode active materials include silver vanadium oxide, copper silvervanadium oxide, manganese dioxide, titanium disulfide, copper oxide,copper sulfide, iron sulfide, iron disulfide, cobalt oxide, nickeloxide, copper vanadium oxide, and other materials typically used inalkali metal electrochemical cells. Carbonaceous materials such asgraphite, carbon and fluorinated carbon, which are useful in both liquiddepolarizer and solid cathode primary cells and in rechargeable,secondary cells, are also useful with the present conductive substrate.

Thus, the present invention further comprises taking about 80 to about99 weight percent of an already prepared electrode active material in afinely divided form and providing a slurry comprising the material.Prior to contact with the grid structure 14 of the conductive substrate10 of the present invention, however, the finely divided electrodematerial is preferably mixed with up to about 10 weight percent of abinder material, preferably a thermoplastic polymeric binder material.The thermoplastic polymeric binder material is used in its broad senseand any polymeric material, preferably in a powdered form, which isinert in the cell and which passes through a thermoplastic state,whether or not it finally sets or cures, is included within the term“thermoplastic polymer”. Representative materials include polyethylene,polypropylene and fluoropolymers such as fluorinated ethylene andpropylene, polyvinylidene fluoride (PVDF) and polytetrafluoroethylene(PTFE), the latter material being most preferred. Natural rubbers arealso useful as the binder material with the present invention.

In the case of a primary, solid cathode electrochemical cell, thecathode active material contacted to the “basket weave” conductivesubstrate is further combined with up to about 5 weight percent of adischarge promoter diluent such as acetylene black, carbon black and/orgraphite. A preferred carbonaceous diluent is Ketjenblack® carbon.Metallic powders such as nickel, aluminum, titanium and stainless steelin powder form are also useful as conductive diluents.

Similarly, if the active material is a carbonaceous material serving asthe cathode current collector in a primary, liquid depolarizer cell or acarbonaceous counter electrode in a secondary cell, the electrodematerial preferably includes a conductive diluent and a binder materialin a similar manner as the previously described primary, solid cathodeelectrochemical cell.

To form the electrode active slurry, about 94 weight percent of thecathode material, regardless of whether it is a carbonaceous material orone or more of a mixture of the other previously described cathodeactive materials, is combined in a twin screw mixer with a dispersion ofabout 0 to 3 weight percent of a conductive diluent, about 1 to 5 weightpercent of a powder fluoro-resin binder and a high permittivity solventsuch as a cyclic amide, a cyclic carbonate or a cyclic ester.

After mixing sufficiently to ensure that the conductive diluent and thebinder material are completely dispersed throughout the admixture and tootherwise completely homogenize the various constituents, the electrodeadmixture is removed from the mixer as a slurry containing about 14%solids, by volume. The step of subjecting the electrode admixture to themixer to form the slurry can also include the addition of a liquidelectrolyte. The electrode admixture slurry has a dough-like consistencyand is preferably contacted onto the opposed sides of the grid structure14 of the conductive substrate 10 of the present invention.

The thusly formed cathode laminate is heated to a temperature of betweenabout 80° C. to about 130° C. and more preferably to about 110° C., fora period of about 30 minutes to about 60 minutes. The heating step ispreferably carried out under vacuum and serves to remove any residualsolvent from the cathode material. Heating, which further serves toplasticize the binder material, helps to ensure the structural integrityof the newly manufactured electrode laminate. The electrode laminate canthen be stored for later use, or is immediately useable in anelectrochemical cell. After drying to remove all residual water from theslurry contacted to the conductive substrate, the resulting anhydrousactive admixture is calendared under a pressure of about 40 tons/inch²to laminate the active admixture to the grid structure 14 of theconductive substrate 10 of the present invention.

An alternate preferred method for providing an electrode is to form theblended electrode active admixture into a free-standing sheet prior tobeing contacted to the grid structure 14 of the conductive substrate 10.One preferred method of preparing a cathode material into afree-standing sheet is thoroughly described in U.S. Pat. No. 5,435,874to Takeuchi et al., which is assigned to the assignee of the presentinvention and incorporated herein by reference. Other techniques forcontacting the active material to the conductive substrate includesrolling, spreading or pressing the admixture thereto.

Cathodes prepared as described above are flexible and may be in the formof one or more plates operatively associated with at least one or moreplates of anode material, or in the form of a strip wound with acorresponding strip of anode material in a structure similar to a“jellyroll”.

The anode is of a metal selected from Group IA, IIA or IIIB of thePeriodic Table of the Elements, including lithium, sodium, potassium,etc., and their alloys and intermetallic compounds including, forexample, Li—Si, Li—Al, Li—B and Li—Si—B alloys and intermetalliccompounds. The preferred anode comprises lithium, and the more preferredanode comprises a lithium alloy such as a lithium-aluminum alloy.However, the greater the amount of aluminum present by weight in thealloy the lower the energy density of the cell.

The form of the anode may vary, but preferably the anode is a thin metalsheet or foil of the anode metal, pressed or rolled on a metallic anodecurrent collector, i.e., preferably comprising nickel, to form an anodecomponent. Preferably, the anode current collector is of the presentconstruction. In the exemplary cell of the present invention, the anodecomponent has an extended tab or lead of the same material as the anodecurrent collector, i.e., preferably nickel, integrally formed therewithsuch as by welding and contacted by a weld to a cell case of conductivemetal in a case-negative electrical configuration. Alternatively, theanode may be formed in some other geometry, such as a bobbin shape,cylinder or pellet to allow an alternate low surface cell design.

An electrochemical cell having an alkali metal-containing electrodeserving as an alkali metal anode, or an alkalated cathode body and acarbonaceous counter electrode according to the present inventionfurther includes a separator provided therebetween. The separator is ofelectrically insulative material, and the separator material also ischemically unreactive with the anode and cathode active materials andboth chemically unreactive with and insoluble in the electrolyte. Inaddition, the separator material has a degree of porosity sufficient toallow flow therethrough of the electrolyte during the electrochemicalreaction of the cell. Illustrative separator materials include fabricswoven from fluoropolymeric fibers including polyvinylidene fluoride,polyethylenetetrafluoroethylene, and polyethylenechlorotrifluoroethyleneused either alone or laminated with a fluoropolymeric microporous film.Other suitable separator materials include non-woven glass,polypropylene, polyethylene, glass fiber materials, ceramics, apolytetrafluoroethylene membrane commercially available under thedesignation ZITEX (Chemplast Inc.), a polypropylene membranecommercially available under the designation CELGARD (Celanese PlasticCompany, Inc.) and a membrane commercially available under thedesignation DEXIGLAS (C. H. Dexter, Div., Dexter Corp.).

The electrochemical cell of the present invention further includes anonaqueous, ionically conductive electrolyte which serves as a mediumfor migration of ions between the anode and the cathode electrodesduring the electrochemical reactions of the cell. The electrochemicalreaction at the electrodes involves conversion of ions in atomic ormolecular forms which migrate from the anode to the cathode. Thus,nonaqueous electrolytes suitable for the present invention aresubstantially inert to the anode and cathode materials, and they exhibitthose physical properties necessary for ionic transport, namely, lowviscosity, low surface tension and wettability.

Suitable nonaqueous electrolyte solutions that are useful in bothprimary and secondary cells having an alkali metal electrode and acounter electrode of a solid material contacted to the grid structure 14of the conductive substrate 10 preferably comprise a combination of alithium salt and an organic solvent system. More preferably, theelectrolyte includes an ionizable alkali metal salt dissolved in anaprotic organic solvent or a mixture of solvents comprising a lowviscosity solvent and a high permittivity solvent. The inorganic,ionically conductive salt serves as the vehicle for migration of thealkali metal ions to intercalate into the carbonaceous material.Preferably the ion-forming alkali metal salt is similar to the alkalimetal comprising the anode. Suitable salts include LiPF₆, LiBF₄, LiAsF₆,LiSbF₆, LiClO₄, LiAlCl₄, LiGaCl₄, LiC(SO₂CF₃)₃, LiO₂, LiN(SO₂CF₃)₂,LISCN, LiO₃SCF₂CF₃, LiC₆F₅SO₃, LiO₂CCF₃, LiSO₃F, LiB(C₆H₅)₄ andLiCF₃SO₃, mixtures thereof. Suitable salt concentrations typically rangebetween about 0.8 to 1.5 molar.

In a liquid depolarizer/catholyte cell, suitable active materials suchas sulfur dioxide or oxyhalides including phosphoryl chloride, thionylchloride and sulfuryl chloride are used individually or in combinationwith each other or in combination with halogens and interhalogens, suchas bromine trifluoride, or other electrochemical promoters orstabilizers.

In other electrochemical systems having a solid cathode or in secondarycells, the nonaqueous solvent system comprises low viscosity solventsincluding tetrahydrofuran (THF), methyl acetate (MA), diglyme, trigylme,tetragylme, dimethyl carbonate (DMC), ethyl methyl carbonate (EMC),1,2-dimethoxyethane (DME), diisopropylether, 1,2-diethoxyethane,1-ethoxy, 2-methoxyethane, dipropyl carbonate, ethyl methyl carbonate,methyl propyl carbonate, ethyl propyl carbonate and diethyl carbonate,and mixtures thereof, and high permittivity solvents include cycliccarbonates, cylic esters and cyclic amides such as propylene carbonate(PC), ethylene carbonate (EC), butylene carbonate, acetonitrile,dimethyl sulfoxide, dimethyl formamide, dimethyl acetamide,γ-butyrolactone (GBL), γ-valerolactone and N-methyl-pyrrolidinone (NMP),and mixtures thereof. In the present invention, the preferred alkalimetal is lithium metal. For a solid cathode, primary cell and asecondary cell, the preferred electrolyte is LiAsF₆ or LiPF₆ in a 50:50,by volume, mixture of PC/DME. For a liquid depolarizer cell, thepreferred electrolyte is 1.0M to 1.4M LiBF₄ in T-butyrolactone (GBL).

The preferred form of a primary alkali metal/solid cathodeelectrochemical cell is a case-negative design wherein the anode is incontact with a conductive metal casing and the cathode contacted to thegrid structure 14 of the conductive substrate 10 serving as the currentcollector according to the present invention is the positive terminal.In a secondary electrochemical cell having a case-negativeconfiguration, the anode (counter electrode)/cathode couple is insertedinto the conductive metal casing such that the casing is connected tothe carbonaceous counter electrode grid structure 14 of the currentcollector 10 according to the present invention, and the lithiatedmaterial is contacted to a second current collector, which alsopreferably has the thicker distal tab portion 38 configuration. Ineither case, the current collector for the lithiated material or thecathode electrode is in contact with the positive terminal pin via alead of the same material as the current collector which is welded toboth the current collector and the positive terminal pin for electricalcontact. In a further embodiment, the cell is built in a case-neutralconfiguration with both the anode and the cathode connected torespective terminal leads insulated from the casing by glass-to-metalseals.

A preferred material for the casing is titanium although stainlesssteel, mild steel, nickel-plated mild steel and aluminum are alsosuitable. The casing header comprises a metallic lid having an openingto accommodate the glass-to-metal seal/terminal pin feedthrough for thecathode electrode. The anode electrode or counter electrode ispreferably connected to the case or the lid. An additional opening isprovided for electrolyte filling. The casing header comprises elementshaving compatibility with the other components of the electrochemicalcell and is resistant to corrosion. The cell is thereafter filled withthe electrolyte solution described hereinabove and hermetically sealedsuch as by close-welding a stainless steel plug over the fill hole, butnot limited thereto. The cell of the present invention can also beconstructed in a case-positive design.

It is appreciated that various modifications to the inventive conceptsdescribed herein may be apparent to those skilled in the art withoutdeparting from the spirit and the scope of the present invention definedby the hereinafter appended claims.

What is claimed is:
 1. A current collector for an electrochemical cell,the current collector comprising: a) a frame having a frame thicknessdefined by spaced apart first and second major frame surfaces extendingto a frame perimeter; b) an interior grid bounded by the frame; and c)at least one first tab extending along a tab length from a proximal tabportion connected to the frame perimeter to a distal tab portion,wherein the first tab comprises: i) the proximal tab portion having afirst thickness defined by spaced apart first and second major proximaltab surfaces aligned perpendicular to the tab length; ii) the distal tabportion having a second thickness defined by third and fourth majordistal tab surfaces aligned perpendicular to the tab length; iii)wherein the first and third major surfaces of the respective proximaland distal tab surfaces are coplanar with the first major surface of theframe, and iv) wherein the second thickness of the distal tab portion isgreater than the first thickness of the proximal tab portion to therebyform a step from the second major surface of the proximal tab portion tothe fourth major surface of the distal tab portion.
 2. The currentcollector of claim 1 wherein the proximal tab portion has a first widthdefined by fifth and sixth minor proximal tab surfaces meeting the firstand second major proximal tab surfaces and wherein a notch extendsinwardly into the first width of the proximal tab portion from at leastone of the fifth and sixth minor proximal tab surfaces so that a secondwidth of the notch is less than the first width.
 3. The currentcollector of claim 1 wherein the grid comprises first and second majorgrid surfaces, and wherein the first major grid surface is substantiallyco-planar with the first major surface of the frame, and wherein thefirst and third major surfaces of the respective proximal and distal tabsurfaces, which are coplanar with the first major surface of the frame,are also coplanar with the first major grid surface of the interiorgrid.
 4. The current collector of claim 1 wherein the interior gridcomprises at least two first grid structures extending to spaced apartfirst and second grid ends joined to the frame at first and second framelocations, and at least two second grid structures extending to spacedapart third and fourth grid ends joined to the frame at third and fourthframe locations, and wherein the at least two first grid structuresintersect the at least two second grid structures at locations interiorof the frame to thereby form grid openings.
 5. The current collector ofclaim 1 wherein the second thickness of the distal tab portion is about0.01 to about 2 millimeters.
 6. The current collector of claim 1 whereinthe grid comprises first and second major grid surfaces, and wherein thefirst major grid surface is substantially co-planar with the first majorsurface of the frame.
 7. The current collector of claim 6 wherein athickness of the interior grid between the first and second major gridsurfaces is less than the frame thickness.
 8. The current collector ofclaim 1 wherein a length of the distal tab portion is from about 5percent to about 95 percent of the tab length.
 9. The current collectorof claim 1 wherein the first thickness of the proximal tab portion isabout 25 percent to about 75 percent that of the second thickness of thedistal tab portion.
 10. The current collector of claim 1 wherein theinterior grid comprises a plurality of substantially parallel first gridstructures extending to and meeting with opposed portions of the frameat first and second frame locations and a plurality of substantiallyparallel second grid structures extending to and meeting with opposedportions of the frame at third and fourth frame locations, and whereinthe first grid structures are aligned at an angle ranging from about 90°to about 15° with respect to the second grid structures.
 11. The currentcollector of claim 1 wherein the frame provides the interior grid havinga shape selected from the group consisting of a circle, a triangle, anoctagon, and a hexagon.
 12. The current collector of claim 1 including asecond current collector, comprising: a) a second frame having a secondframe thickness defined by spaced apart third and fourth major framesurface extending to a second frame perimeter; b) a second interior gridbounded by the second frame; c) at least one second tab extending alonga second tab length from a proximal second tab portion connected to thesecond frame perimeter to a distal second tab portion, wherein thesecond tab comprises: i) the proximal second tab portion having a thirdthickness defined by spaced apart seventh and eighth major proximal tabsurfaces aligned perpendicular to the second tab length; ii) the distalsecond tab portion having a fourth thickness defined by ninth and tenthmajor distal tab surfaces aligned perpendicular to the second tablength; iii) wherein the seventh and ninth major surfaces of therespective proximal and distal second tab surfaces are coplanar with thethird major surface of the frame, and iv) wherein the fourth thicknessof the distal second tab portion is greater than the third thickness ofthe proximal second tab portion to thereby form a step from the eighthmajor surface of the proximal second tab portion to the tenth majorsurface of the distal second tab portion; and d) a tab junction joiningthe first tab to the second tab so that the first and second currentcollectors are opposed to each other.
 13. The current collector of claim12 wherein the proximal second tab portion has a third width defined byeleventh and twelfth minor proximal tab surfaces meeting the seventh andeighth major proximal tab surfaces and wherein a second notch extendsinwardly into the third width of the proximal second tab portion from atleast one of the eleventh and twelfth minor proximal tab surfaces sothat a fourth width of the second notch is less than the third width.14. An electrochemical cell, comprising: a) a casing; b) a firstelectrode comprising a first electrode active material supported on afirst current collector; c) a second, counter electrode comprising asecond electrode active material supported on a second currentcollector; d) a separator disposed between the first and secondelectrodes to thereby form an electrode assembly housed inside thecasing, wherein the separator prevents direct physical contact betweenthe electrodes while providing for ionic conductivity through theseparator; e) wherein at least one of the first and second currentcollectors comprises: i) a frame having a frame thickness defined byspaced apart first and second major frame surfaces extending to a frameperimeter; ii) an interior grid bounded by the frame; and iii) at leastone tab extending along a tab length from a proximal tab portionconnected to the frame perimeter to a distal tab portion, wherein thetab comprises: A) the proximal tab portion having a first thicknessdefined by spaced apart first and second major proximal tab surfacesaligned perpendicular to the tab length; B) the distal tab portionhaving a second thickness defined by third and fourth major distal tabsurfaces aligned perpendicular to the tab length; C) wherein the firstand third major surfaces of the respective proximal and distal tabsurfaces are coplanar with the first major surface of the frame, and D)wherein the second thickness of the distal tab portion is greater thanthe first thickness of the proximal tab portion to thereby form a stepfrom the second major surface of the proximal tab portion to the fourthmajor surface of the distal tab portion; and f) an electrolyte residingwithin the casing activating the first and second electrodes.
 15. Theelectrochemical cell of claim 14 wherein the proximal tab portion has afirst width defined by fifth and sixth minor proximal tab surfacesmeeting the first and second major proximal tab surfaces and wherein anotch extends inwardly into the first width of the proximal tab portionfrom at least one of the fifth and sixth minor proximal tab surfaces sothat a second width of the notch is less than the first width.
 16. Theelectrochemical cell of claim 14 wherein the first thickness of theproximal tab portion is about 25 percent to about 75 percent that of thesecond thickness of the distal tab portion.
 17. The electrochemical cellof claim 14 wherein the electrolyte includes a first solvent and asecond solvent, and wherein the first solvent is selected from the groupconsisting of tetrahydrofuran, methyl acetate, diglyme, triglyme,tetraglyme, 1,2-dimethoxyethane, diisopropylether, 1,2-diethoxyethane,1-ethoxy, 2-methoxyethane, dimethyl carbonate, diethyl carbonate,dipropyl carbonate, ethyl methyl carbonate, methyl propyl carbonate andethyl propyl carbonate, and mixtures thereof, and the second solvent isselected from the group consisting of propylene carbonate, ethylenecarbonate, butylene carbonate, acetonitrile, dimethyl sulfoxide,dimethyl formamide, dimethyl acetamide, γ-valerolactone, γ-butyrolactoneand N-methyl-pyrrolidinone, and mixtures thereof, and wherein theelectrolyte includes an alkali metal salt selected from the groupconsisting of LiPF₆, LiBF₄, LiAsF₆, LiSbF₆, LiClO₄, LiAlCl₄, LiGaCl₄,LiC(SO₂CF₃)₃, LiO₂, LiN(SO₂CF₃)₂, LiSCN, LiO₃SCF₂CF₃, LiC₆F₅SO₃,LiO₂CCF₃, LiSO₃F, LiB(C₆H₅)₄ and LiCF₃SO₃, and mixtures thereof.
 18. Theelectrochemical cell of claim 14 wherein the first electrode is an anodeand the first electrode active material is comprised of lithium or alithium-aluminum alloy, and wherein the second electrode is a cathodeand the second electrode active material is selected from the groupconsisting of silver vanadium oxide, copper silver vanadium oxide,copper vanadium oxide, vanadium oxide, manganese dioxide, titaniumdisulfide, copper oxide, copper sulfide, iron sulfide, iron disulfide,cobalt oxide, nickel oxide, graphite, carbon, fluorinated carbon, andmixtures thereof.
 19. The electrochemical cell of claim 14 wherein theat least one of the first and second current collectors is selected fromthe group consisting of titanium, molybdenum, tantalum, niobium, cobalt,nickel, stainless steel, tungsten, platinum, palladium, gold, silver,copper, chromium, vanadium, aluminum, zirconium, hafnium, zinc, iron,and mixtures and alloys thereof.
 20. The electrochemical cell of claim14 being of either a primary or a secondary chemistry.